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57) what is the probability?

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by sanju09 » Wed May 12, 2010 3:24 am
ern5231 wrote:a and b are selected from the set {1,2,3,4,5}. What is the probability that ab<4?
Selection can be made in 5C2 = 10 ways, and the favorable ways for a, b such that a b < 4, are

1 & 2 and 1 & 3, only 2, order doesn't matter here

the required probability is 2/10 = [spoiler]1/5 = 0.2[/spoiler].
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by GauravMalhotra » Wed May 12, 2010 3:25 am
Total number of ways in which 'a' can be selected is 5.
Total number of ways in which 'b' can be selected is 4.

so total 20 ways.

'ab' can be < 4 by following

a = 1 b=2
a = 1 b=3
a = 2 b=1
a = 3 b=1

total 4 ways...

answer = 4/20 or 1/5

thanks
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